1. Field of the Invention
The present invention relates to all-optical OR and XOR logic elements, and more particularly, to all-optical OR and XOR logic elements employing saturable absorbers as optical gates.
2. Description of the Prior Art
Generally, a computing system is constructed by integrating gates such as AND, OR, XOR, NAND, NOR, NXOR, etc. Conventional computing systems have processed electrical signals using silicon-based devices. The conventional computing systems have restrictions on a processing speed and a capacity because needs for high speed and large capacity of the systems have been recently increased. To overcome the restrictions, a computing system having optical elements excellent in the processing speed or the capacity has been developed.
In particular, a number of researches and developments have been made on OR and XOR logic elements such as an XOR gate using an ultra-fast non-linear interferometer (UNI) disclosed in xe2x80x9c20 Gbps All-optical XOR with UNI gatexe2x80x9d, IEEE Photonic Technol., Vol. 12, pp. 834-836, (2000), by C. Bintjas et al., an XOR gate using a Sagnac gate disclosed in xe2x80x9c10 GHz Boolean XOR with semiconductor optical amplifier fiber Sagnac gatexe2x80x9d, CLEO, CThF5 (1999), by K. Zoiros et al., an OR gate using an interferometric wavelength converter disclosed in xe2x80x9c10 Gbps All-optical logic OR in monolithically integrated interferometric wavelength converterxe2x80x9d, Vol. 36, IEEE Electron. Lett., pp. 813-815 (2000), by T. Fjelde et al., an XOR gate disclosed in xe2x80x9cDemonstration of 20 Gbps All-optical logic XOR in integrated SOA-based interferometric wavelength converterxe2x80x9d, Vol. 36, pp. 1863-1864, (2000), by T. Fielde et al., and so forth.
Although the XOR gate using an ultra-fast nonlinear interferometer (UNI) and the XOR gate using a Sagnag gate have an advantage of high speed, they have also some disadvantages that it is difficult to adapt them to optical computing systems requiring a high degree of integration, due to the complexity of their essential elements consisting of optical fiber and the difficulty in integration with other elements.
Meanwhile, a semiconductor optical amplifier (SOA) uses a gain characteristic of semiconductor to amplify an input optical signal by its gain. Also, the semiconductor optical amplifier directly amplifies the optical signal without converting the optical signal into an electrical signal. Further, the semiconductor optical amplifier is constructed to have a structure similar to a semiconductor laser using compound semiconductor materials, and is subjected to a anti-reflecting thin film processing. For this reason, the semiconductor optical amplifier can amplify the optical signals by a high gain in a wide wavelength range in an optical communication system having wavelength band of 1.55 xcexcm. Furthermore, the semiconductor optical amplifier has a smaller size than a conventional erbium-doped amplifier (EDFA) and can be monolithically integrated with other semiconductor optical elements and passive waveguides. Accordingly, the semiconductor optical amplifier is useful in various fields of applications such as a wavelength converter, an optical switch, a logic element, etc.
Now, the conventional all-optical OR and XOR logic elements using a semiconductor optical amplifier will be described with reference to FIGS. 1A and 1B.
The element shown in FIG. 1A includes first and second optical amplifiers 7 and 8, a Y-combiner 4, and a Y-branch 5, whereas the element shown in FIG. 1B includes first and second optical amplifiers 17 and 18, Y-combiners 14a, 14b and 15b, a Y-branch 15a and a filter 19.
The logic element in FIG. 1A constitutes a Michelson interferometer, whereas the logic element in FIG. 1B constitutes a Mach-Zehnder interferometer. In the Michelson interferometer, a continuous-wave signal of xcexcw is splitted by a Y-branch 5 and then inputted to the first and second optical amplifiers 7 and 8, respectively, whereas input optical signals of xcexs1 and xcexs2 are inputted to a first optical amplifier 7. In this case, the surface to which the continuous-wave signal of xcexcw is provided with a anti-reflecting thin film 9, while the surface to which the input optical signals of xcexs1 and xcexs2 is provided with a reflection facet 6. It is known that if a surface is cleaved, that is, if a surface is not provided with the anti-reflecting thin film 9, its reflectivity reaches about 30%.
The Mach-Zehnder interferometer in FIG. 1B is distinguished from the Michelson interferometer in FIG. 1A in that anti-reflecting thin film 16a and 16b are deposited on both surfaces. That is, the Michelson interferometer as well as the Mach-Zehnder interferometer makes use of a cross-phase modulation, and the two interferometers are similar to each other in that the phase of the continuous-wave signal is varied depending on the optical power of an input optical signal when a semiconductor optical amplifier is gain saturated. However, the two interferometers are different from each other in that in the Michelson interferometer, one surface is designed to be reflective so that the total length of the element is reduced to a half. That is, the size of the Mach-Zehnder interferometer can be reduced to a half using the reflecting surface because of its symmetrical configuration.
The interferometers described above make use of the phase difference of two paths. Therefore, non-linear materials such as an optical amplifier play an important role. Also, in order to obtain a sufficient phase shift by xcfx80, a variety of methods of, e.g. lengthening an optical amplifier, significantly increasing amount of input current to the optical amplifier, or significantly increasing an input optical power are used. Although lengthening an optical amplifier is the easiest way among them, the method is restricted to some extent since the total length of the element is increased with increase of the length of the optical amplifier. The length of the Michelson interferometer can be decreased to a half of the length of the Mach-Zehnder interferometer because it uses the reflecting surface as described above. That is, since the input optical signal is reflected and returns from the reflecting surface, the input optical signal passes through the optical amplifier twice. Nevertheless, since the input continuous-wave signal and the output logic signal are outputted from the same surface in the Michelson interferometer, an expensive device such as a circulator is additionally required to divide the two signals.
The conventional Mach-Zehnder interferometer type OR and XOR logic elements using the semiconductor optical amplifier uses a cross-phase modulation of the semiconductor optical amplifier. That is, the Mach-Zehnder interferometer takes advantage of the fact that the phase of the continuous-wave signal is varied depending on the optical power of the input optical signal in a state of the gain saturation of the semiconductor optical amplifier. When a cross-phase modulation is used, input current to each semiconductor optical amplifier is differentiated to increase the phase difference between the paths A and B.
Therefore, conventionally, since phase of the continuous-wave signal is varied depending on the intensity of the input optical signal and a desired operational characteristic can be obtained only in a specific range of the intensity of the input optical signal and only with a specific input current to the semiconductor optical amplifier (SOA), an operational range was very restrictive.
It is an object of the present invention to alleviate a restriction on the operational input power dynamic range by making no phase difference between paths be generated depending upon the variation of an input optical power, unlike the cross-phase modulation type logic element using a semiconductor optical amplifier.
Another object of the present invention is to provide a logic element having available advantages such as elimination of noises and increase of extinction ratio.
In order to accomplish the above objects, the present invention provides a logic element comprising: a first saturable absorber for receiving a combined power of a first input optical signal and a reference signal, its passing power being higher than its absorbed power if the combined power is higher than a first transparent input power and its absorbed power being higher than its passing power if the combined power is lower than the first transparent input power level; a second saturable absorber for receiving a combined power of a second input optical signal having a wavelength different from that of the first input optical signal and the reference signal, its passing power being higher than its absorbed power if the combined power is higher than a second transparent input power and its absorbed power being higher than its passing power if the combined power is lower than the second transparent input power; and a combiner for combining an output of the first saturable absorber and an output of the second saturable absorber, wherein the reference signal has an optical power lower than the first and second transparent input powers, and the combined power of the first input optical signal and the reference signal and the combined power of the second input optical signal and the reference signal are higher than the first and second transparent input powers, respectively.
Further, the logic element may further comprise a phase shifter arranged between the first saturable absorber and the combiner or between the second saturable absorber and the combiner, an XOR logic element is implemented when the phase shifter generates a phase difference by xcfx80, and an OR logic element is implemented when the phase shifter generates a phase difference by zero
Furthermore, the reference signal inputted to the first saturable absorber and the second saturable absorber is generated by dividing a continuous-wave signal into two half signals having the same power and the two half signals are inputted to the first and second saturable absorbers, respectively.
Furthermore, the logic element may further comprise an optical amplifier provided at an output end of the combiner, the gain of optical amplifier is saturated or output power of the optical amplifier is kept constant when the input optical signal has an optical power higher than a saturation input optical power, wherein a combined power Ptr,out of the first and second transparent output optical powers of the respective first and second saturable absorbers is higher than the saturation input optical power.